Doctor blade pressure control system and method



Sept. 4, 1951 G. STEVENSON DOCTOR BLADE PRESSURE CONTROL SYSTEM AND METHOD Filed May 17, 1949 will INVENTOR.

Gustz; Szvcnson 03. JW fag- Patented Se t. 4, 1951 SYSTEM AND METRO Gustie Stevenson, Philadelphia, Pa., assignor to F. .I. Stokes Machine Company, a corporation of Pennsylvania Application May 11, 1949, Serial a... 93,809

9 Claims.

This invention relates to a method of and apparatus for controlling the pressure applied to doctor knives for removing material from the surface of rotating drums, such as used in drum driers. Specifically the invention relates to a method and control apparatus for al p y fl a predetermined pressure to a doctor knife and cyclically and periodically relieving this pressure by a predetermined amount.

The invention is especially applicable to drum drier wherein a heated drum receives a thin film coating of a liquid slurry which is dried (the water content is reduced by evaporation) in less than one revolution of the drum. It is also applicable to flaking drums, which are cooled by water or other refrigerant and which, by several available methods of feed. receive a thin film coating of a very hot compound (for instance pitch compounds at 550 1'.) which is dried to brittleness by extracting heat from the com pound. The invention is useful in any situation where a doctor blade is used to remove material from the surface of a rotating drum.

Generally, knife pressure against the drum is obtained by a combination of levers and weights and/or springs or by pressure screws. when the proper knife pressure ha been determined the pressure setting is not changed so long as 2 heretofore been very dimcult to doctor satisfac- Another object of the invention is to provide a controlapparatus which will periodically re- .lieve the knife pressure by a predetermined amount.

Still another object is to provide a control apparatus wherein the original pressure, the degree of relief, and the duration of the periods of application and relief may be varied to provide the most eiilcient combination for thematerial being doctored.

A further object is to provide a control apparatus having provision for-raising the knife from the surface of the drum.

The accompanying drawing diagrammatically represents a preferred embodiment of the invention utilizing fluid-pressure operatedcylinders, controlled by two timers, for varying the 2 pressure on the knife.

Referring to the drawing. l is a rotating drum such as is used in drum driers. and is mounted on shaft Ia which is iournaled in suitable bearings not hown. The doctor blade or knife 1 "is carried by shaft arranged in parallel relathe film of material is cleanly doctored from i the drum surface. This may last for an indefinite or extended period. However, experience has shown that some materials will cling so tenaciously to the surface of a drum that when a constant unvarying knife pressure is maintained, the hard film will slip under the knife and will "not be scraped or doctored oil of the surface.

Worse yet, the drum will receive another coating of material on top of the previous one and the pernicious cycle repeated until eventual complete failure of the process.

The present invention is based on the discovery that by relieving the knife pressure momentarily or for a short period of time and then reapplying the original higher pressure, the knife will continue to doctor the drum cleanly. The amount of original pressure, the degree to which it is relieved, and the durations of application and relief will depend upon the characteristics of the material, the speed of rotation of the drum. and other variables which must be determined for each individual case.

A primary object of the invention is to provide a novel method of doctoring materials from a drum which may be maintained at a high detion with shaft la and journaled at each end in suitable bearings not shown. The blade or knife} 2 may be raised from or lowered to the surface of drum I by rotation of shaft 2a. This is accomplished by means of double-acting air cylinder 5 connected through piston rod 4 to actuating arm 3 which is keyed to the haft 2a.

Compressed air for operating cylinder i is supplied through line 6 to three pressure reduction valves 1, 8 and 9. These reduction valves are set to supply air at three different pressures for reasons that will soon be apparent and each value is adjustable to provide any desired pressure within certain limits. For purposes of illustration it will be assumed that valve TI reduces the air pressure to lbs/sq. in. (low pressure); valve 8 to lbs./sq. in. (intermediate pressure) valve 9 to 80 lbs/sq. in. (high pressure).

Pressure regulating valves 8 and 9 are selectively connected to the front end of cylinder 5 through a two-position valve to and a cut-oi! valve I I connected in series, and regulating valve 1 is connected to the rear end of cylinder 5 through a solenoid operated valve If. These solenoid operated valves are of known construction and the diagrammatic showing in the drawing is for the purpose of illustration only. The

inlet port I20. of valve I2 is connected to regulating valve 1, and the outlet port lZb is connected to the rear end of cylinder 5. Reduction valve I Isconnectedtoinletport Ilbofsolenoidvalve II and reduction valve 8 is connected to inlet port Ita oi the same valve. Outlet ports Ito and Ilid oi solenoid valve It are connected to inlet port Ilc oi solenoid valve II. Gheck valves Ito and no are inserted in the lines from outlet ports I lie and "Id to prevent any air from flowing from solenoid valve II back to solenoid valve III or between ports Ito and Id. Exhaust ports He and I2c of solenoid valves II and I2, r pectively, are connected to an atmospheric exhaust; outlet port Nb of valve II is connected to the front end of cylinder 5.

Solenoid valve ill when de-enersized is biased by spring we so that ports Illa and Inc are connected by an internal passage to establish a connection from high pressure valve 9 to control valve II. when valve III is energized this connection is interrupted and a connection is established between intermedia pressure valve 8 to control valve II by an internal passage connecting ports lb and Ind. Solenoid valve II when de-enerized is biased by spring IId so that ports IIb and He are connected by an internal passage to provide an exhaust connection for the front end of cylinder 5, and when valve II is energized ports Na and b are connected to'complete a connection from valve In to cylinder 5. Similarly, valve I2 is biased by spring I 2d so that ports I2b and |2c are connected to establish an exhaust connection for the rear end of cylinder 5 when the valveis de-energized. and ports I2a and I2b are connected when the valve is energized to establish the connection from the low pressure valve 1 to the rear end of cylinder 5. In all cases the energized and dc-energized" connections as described above are mutually exclusive, i. e. they cannot exist simultaneously in the same valve at the same time.

Connected to the line between outlet port Ilbh I noid valves is supplied from circuit I6 to lines Ito and IE2) through main switch II. The energizing circuit of valve I0 is controlled by two electric timers I8 and I9 connected to lines Ito and IBD and to each other in such a manner that they will operate sequentially, i, e. when timer I9 has timed out the delay period for which it is set it will start timer I8v and when timer It has timed out the period for which it is set it will restart timer I9. This alternate operation of timers will continue as long as the timers are supplied with current, and since each timer is minutely adjustable within its range, an infinite number of time-delay combinations is available. These timers are of conventional construction, such as electric timer type 30HLI made by Photoswitch, Inc., and need not be described in detail. It is suflicient to say that timer I8 has a set of contacts which are closed while the timer is in operation, and thse contacts supply current to output terminals, I80 and I8b 'to which the solenoid of valve III is connected. Thus current will be supplied to valve It for intermittent intervals, the duration of these intervals being determined by the setting of timer II and the duration of the intervening periods being determined by the setting of timer II.

I timers It and l9- or to solenoid valve I2. The

coil of switch 20 is connected in parallel with the solenoid of valve In and is energized during the operation of timer It.

The solenoid of valve I2 is connected directly across the lines Ito and I62; and will be energized whenever main switch I! is closed to apply low valve II.

pressure to the right end of cylinder 5.

When the main switch I! is open as shown in the drawing the high pressure air is admitted to valve III and check valve I 3a but is cut oif at valve II; the intermediate pressure air is cut oil at valve III; the low pressure air is cut off at valve I2; and both ends of the cylinder 5 are connected to atmosphere. To place the system in operation main switch I! and release switch 2| are closed. This will energize valves II and I2 and start timer I9. The energization of valves II and I2 disconnects the two sides of the cylinder 5 fromthe atmospheric exhaust and allows high pressure air to flow to the left end and low pressure air to flow to the other; the resultant differential pressure (30 lbs/sq. in.) causes the doctor blade 2 'to be pressed against the surface of drum I. The contacts of pressure witch I4 will be opened at this time since the pressure in the line exceeds 65 lbs/sq. in.

As soon as timer I9 has timed out the delay for which it is set, timer I8 will be started and current supplied through terminals I8a and "lb to valve II) and the contacts of relay 20. The energization of valve II'I causes the high pressure air to be cut oil and allows intermediate pressure air to flow through the valve and check valve lib to valve II. Simultaneously \vith the energization of valve- III the contacts of relay 2f are opened and since the contacts of switch It are also open due to the high pressure in the line, valve II is de-energized. When this happens the high pressure side of cylinder 5 is again connected to the atmospheric exhaust, allowing the pressure to drop until it reaches '60 lbs/sq. in. when the contacts of pressure switch It will close, shorting out the open contacts of switch 20 and energizing At this point a new condition of balance will be attained in which the intermediate pressure air is connected to the cylinder 5 through valves I0 and II and the resultant differential pressure acting on the piston of cylinder 5 is reduced to 10 lbs/sq. in. causing a reduction in the force with which the doctor blade 2 is pressed against the surface of the drum I.

when the timer I8 has timed out the interval for which it is set, the current supply to terminals Ito and lab will be interrupted and timer I9 will be re-started. Valve III will be deenergized and high pressure air will again be admitted to the cylinder 5 through check valve Ito and valve ,II. .At the same time the contacts of switch 20 will be closed so that when the contacts of switch It are opened by the increase in pressure the valve II will not be deenergized. Thus the system is returned to its original condition and is ready for repetition of the above described cycle.

As already explained, the frequency of applying and relieving the pressure on the doctor blade 15 will depend upon the nature or the material being doctored. The usual speed of rotation of a drum drier is from 2 to 8 revolutions per minute, and when using timers which are adjustable from zero to 4 minute intervals, it is possible to adjust the frequency of the control cycle to any value between many times per revolution of the drum to only once every 32 revolutions of the drum.

A satisfactory explanation of why the periodic release of the blade pressure causes improved doctoring has not been developed, but actual experience has demonstrated that when the blades -'cease to clean the drum surface, relieving the pressure and then reapplying it causes them to bite into the baked on material and to cut through it. It seems possible that the blade is somewhat flexed when under normal pressure, and when this pressure is released, the blade straightens out somewhat to increase the angle of attack and this causes it to dig into the hard coating.

The release switch 2| is for the purpose of raising the doctor blade 2 clear of the drum I when desired. Whenthe switch 2| is opened the valve II will be de-energized and the left or high pressure side of the cylinder 5 will be connected to theatmospheric exhaust; valve l2, however,will not be aflected and the low pressure air will continue to be supplied to the other side of cylinder 5, with the result that air under pressure of 50 lbs./sq. in. will be applied to the right end of cylinder 5 and the doctor blade 2 will be raised clear of the drum I.

It will be apparent to those skilled in the art that other connections than those described in this illustrative example are possible without departing from the spirit of my invention. In the arrangement shown, if the electric power should fail, all valves will be moved to a position to cut oil the compressed air and to conduci'keach end of cylinder 5 to exhaust. While I prefer to use two cylinders, one connected to each end of the doctor blade, only one may be used with operating means to apply equalized pressures to the two ends of the blade.

What I claim is: I

1. A pressure control system for adoctor blade associated with a rotary drum comprising a douhie-acting fluid-pressure actuated cylinder connected to said blade to move the blade towards and away from said drum, a source of fluid under pressure, means energizing said cylinder from said source to press said blade against said drum with a predetermined pressure, and means acting periodically at regular intervals for reducing said pressure to a lower value while maintainins said blade in contact with said drum.

2. A pressure control system according to claim 1 and including manually operable means for energizing said cylinder from said source of fluid to move said blade away from said drum.

3. A pressure control system for a doctor blade associated with a rotary drum comprising a double-acting fluid-pressure actuated cylinder connected to said blade-to move the blade towards and away from said drum, a source of fluid at high, intermediate and low pressures, a connection from the low pressure source to one end of said cylinder for moving said blade away from said drum, a connection from the high pressure source to the other end of said cylinder whereby said blade is pressed against said drum-by a thehighandlowpressures,aidmeansacting periodically at regular intervals for switching said last named connection from said high pressure source to said intermediate pressure source and back to said high pressure source.

4. A pressure control system according to claim 3 and including manually operable means for cutting off the second named connection whereby said low pressure source moves said blade away from said drum. I v

5. A pressure control system for the doctor blade of a rotary drum comprising a fluid pressure actuated cylinder connected to said blade to press the blade against said drum, a source of fluid at high pressure and a source at intermediate pressure, means including a two-position valve for connecting one or the other of said sources to said cylinder, and means for periodically operating said valve from one pomtion to the other.

' 6. A pressure control system according to claim 5 wherein said valve is normally biased to a position connecting the high pressure source to the cylinder and is provided with a .magnet for operation to the other position, and including a cut-o'fi' valve included in the connection between said two-position valve and said cylinder and being normally biased to cut-offposition, a normally closed pressure-actuated switch connected between said cut-off valve and said cylinder and being operable to open position at a pressure between said high and intermediate valves, an energizing circuit for said cut-off valve completed through said pressure switch. and a control relay energized in parallel with the magnet of said two-position valve and having normally closed contacts connected across the contacts or said pressure switch.

8. A pressure control system fora doctor blade on a rotary drum comprising means for pressing said blade against said drum at a predetermined pressure determined by the difference between GUBTIE STEVENSON.

nnnnnnncns crrnn I The following references are of record in th file of this patent:

UNITED STATES PATENTS Number Name Date 1,704,853 Berry Mar. 12, 1929 2,330,889 Holcomb Oct. 5, 1948 2,474,052

Johnson June 21, 1949 

